Summary The insulin-like growth factors play important roles in normal growth and development. It is widely expressed in a tissue-and developmental-specific fashion, and has different functions in the various tissues and organ systems. The gene encoding IGF-I is therefore, complex with multiple controls at the transcription, translation and post- translational levels. There are two basic promoters with unique features and the alternative usage of each promoter results in differential usage of leader exons with different leader peptides being expressed. In addition, alternative splicing of 3' exons results in different E peptides who may undergo different post-translational (glycosylation) modifications. These differences may play a role in the differential expression and function of IGF-I in normal physiology and disease states. The IGFs are also important factors in autocrine/ paracrine-induced tumor growth. Breast cancer growth is enhanced by IGFs. This effect as well as the inhibition of growth by retinoic acid, is associated with modification of the release of IGF binding proteins (IGFBPs). Some IGFBPs may inhibit growth, whereas, other may enhance. Thus, the IGFBPs play an important modulatory role in cancer growth.